As a system for effecting the vector control of an ac induction motor with a high degree of accuracy, there has been known a system including a closed loop so that the exciting current component and torque current component in a rotating magnetic field coordinate system of a motor are detected and the detected values are fed back to a primary voltage command generating unit, thereby controlling the primary voltage value to reduce the deviations between these detected values and an exciting current command value and torque current command value, respectively, to zero. In the motor of the type driven by a power converter, e.g., an inverter, the primary current contains much harmonic components. With the conventional vector control system, the detection of the primary current of a motor is effected in such a manner that in order to avoid any fundamental harmonic current detection error due to the harmonic components, the primary current value is sampled by a high-speed A/D converter having a sampling frequency (e.g., a sampling period of 1 to 10 .mu.sec) which is about 10 times the harmonic frequency and an exciting current component i.sub.d and a torque current component i.sub.q are calculated from the sampled value. Therefore, it is necessary that the processor used for signal processing purposes must also be of a high-speed type having a processing speed which is about the same as the sampling period of the A/D converter. This is a cause of preventing all the circuits of the control system from being processed digitally by the microprocessor.
An example of a vector control system for an induction motor driven by a VVVF inverter is disclosed in JP-B-No. 60-19236 filed in Japan by Toyo Denki Seizo Kabushiki Kaisha on Apr. 15, 1980. With this system, in order to eliminate the harmonic components, the sampling frequency of the primary current must be set greater than two times the harmonic frequency and a high-speed A/D converter and processor are required.